Békollè, David
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Békollè, David
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Békollè, David
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Item type:Publication, Wireless Mesh Network : a rural community caseWe observe an emphasis on deploying wireless networks in a mesh topology, which is a cost-effective approach to extend their coverage, since a decade. But this approach of planning a network is lowly adopted in Africa, especially in rural regions, despite the suitability of this type of network for bridging the digital divide. In this thesis, we tackle the planning problem of wireless mesh networks in rural regions: How they can be deployed in rural communities using a landline node such as a telecentre at low cost. Since this problem is not only a technical one, it is important to study the feasibility of such a planning process. We consider two rural communities in Cameroon that are hosting a telecentre as a case study for this purpose. Then we investigate, which wireless technology could be appropriate, and discuss the propagation of the signal in rural regions. After justifying the choice of the 802.11n wireless network, empirical studies are conducted in different scenarios: free space, built-up areas, and wooded areas. Different path loss models are compared and the best one is improved. The main part of this work deals with the placement problem of mesh routers in a given rural region hosting a predefined Internet gateway. This issue is a critical one, especially in rural regions where we usually observe low density and sparse population. We suppose the planning of wireless mesh networks to be coverage-driven, meaning that we are more concerned about the space to cover than about the capacity to provide. We provide a network model tied to rural regions by considering the area to cover as decomposed into a set of elementary areas. Each elementary area may be of different types: an area of interest in terms of coverage, an area representing an obstacle, an area where a node can be placed or an area requiring a lower settlement cost. The placement problem is therefore considered as a multi-objective optimisation problemwith conflicted objectives such as maximising the coverage while minimising the cost. To solve this problem, we consider at first the case where the area to cover is continuous. We propose different algorithms based on simulated annealing. The first onemaximises the coverage while reducing the number of routers; the second one improves the cost by relocating nodes into cost-effective locations. A set of trade-offs resulting from a Pareto optimisation is produced. The case where areas of interest are disjoint is considered later. The approach proposed to solve this new model reuses the previous optimisation algorithms and adapts some algorithms used in graph theory, such as Depth First Search, Breath First Search, closest pair, and Graham Scan for Convex Hull. They are used in order to detect sub-networks and to link them efficiently. Finally, the problem of channel assignment is addressed. After assimilating this problem to the one of list edge colouring, a strategy aiming to assign channels by creating a spanning tree from the sole Internet gateway is proposed. All proposed algorithms are evaluated on different regions. The results show a certain convergence and an efficiency of the approach despite the probabilistic nature of underlying algorithms.doctoral thesis490 192 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, A Malware Detection System For Android(2015-09-16); ; ; Android security is built upon a permission-based mechanism, which restricts access of third-party Android applications to critical resources on an Android device. The user must accept the set of permissions that an application requires, before the installation proceeds. This is to inform the user about the risks of installing and using an application. It has two problems. The first one is that users are not aware enough of existing threats and trust either the application store or the popularity of the application and accept the installation without analysing the intentions of the developer. The second one is that Android does not display the specific resource needed by the application and the corresponding permissions during its installation. It rather presents different categories representing the set of resources with a description. The categories include implicitly permissions necessary to access some resources. The user grants more authorisations than necessary probably confused by the management of permissions, increasing the difficulty of detecting malicious applications and constituting the basis for many attacks. The thesis defines a system for detecting Android malware based only on requested permissions. It focuses on 222 permissions including some exclusively for third-party applications. It is a static analysis technique, which combines two reliable strategies. The the first one focuses on the discriminating metric based on the frequency of permissions and the proportion of requests by malicious applications within the whole sample. The second one relies on security risks related to granting permissions. A comparison has shown that the four protection levels of permissions defined by Google are coarse-grained, hiding the real sense of permissions. The first strategy is fine-grained and more precise in terms of permission semantics. We collected a dataset with 6783 malicious and 1993 normal applications, which have been tested and validated. Profiles for each sample have been generated, depending on both strategies and used as input for training and learning processes. Seven classifiers have been applied to the models to output performance results. We select the good ones to define our classifier, which provides outstanding performance in detection and prediction. A dataset of associations of permissions to weights that can be reused in a different research has been released from our work. Evaluations indicate that our model is one of the best tools with only requested permissions as a feature. It is able to detect around 99.20% of 1260 cases of malware released by the Genome project, which represents behaviour of nowadays malware. This work provides a scheme for weighting permissions possibly applicable to an unknown samples dataset, while keeping a good performance in classification. The model is good in detecting Android malware with around 97% of the True Positive Rate and predicting Android malware with around 95% of the True Positive Rate. This means that it is capable to discriminate almost all cases of malware in detection and prediction. The Area Under Curve (AUC) metric is between 97% and 99%, which confers the outsatnding property of the outstanding detection system for the detection of malware. We propose additionally a system that can be embedded into an Android hand-held device for real-time detection. The results of the comparison to three renowned antiviruses reveal that our framework clearly outperforms two of them.doctoral thesis460 636